The moment school protective officer Michael Ortega receives a call is the moment he starts planning what he's going to do next.
"And the situation like this where it's an active shooter, I could be going in alone," Ortega said. "That's the whole purpose of an active shooter situation is to get there and stop the threat."
That's an entirely different tactic than what was used during Columbine. Twenty years ago, police waited for a SWAT team and in those minutes the shooters kept killing. Now, in Colorado, officers are told to go in alone and get the shooter.
The Denver-area manhunt for athis week revived fears of a new attack, nearly 20 years after 12 students and a teacher were killed there. Since Columbine, 122 people have died in mass school shootings across the country.
CBS News' Barry Petersen got rare access to the security preparations, including a sophisticated simulator for police training in the school district, designed to prevent another shooting at Columbine.
Officers are taught this new strategy in the hallways of a former elementary school under the watchful eye of Arvada Police Sgt. Michael Touchton.
"Can I make a difference? Can I save lives? And finally, is it worth it?" Touchton said. "And you gotta put yourself in the mindset that you're gonna go forward and you're gonna save innocent lives."
A classroom with its eerie cutouts of kids, some with guns, is part of that training overseen by John McDonald, executive director for school safety in the district that includes Columbine. Today, there are cameras everywhere monitored 24-7, doors that can be locked remotely and armed officers inside the schools.
"We can pick up a suspect on camera. They can send me a picture … so as we arrive, we don't have to try and figure out who the bad guy is or good guy is," McDonald said.
On that day, police were hampered because they had no idea of the school's layout. So now, McDonald carries a blueprint of every school in his district. There's a state website called Safe2Tell for anonymous tips and social media is monitored for any suspicious posts.
"If you say you're gonna kill us, I believe you. If you say you're gonna blow us up. We believe you. We are gonna do everything in our power to make sure that that doesn't happen," McDonald said.
At the memorial dedicated to those who were killed, Petersen spoke with Columbine seniors Rachel Hill and Teagan Simons, born after the shooting, who grew up in a time when active shooter drills start in kindergarten.
Hill said she feels "extraordinarily safe" at Columbine and that the active shooter drills "aren't overkill."
"I think it's made it easier to really focus on school," Simons said. "Because I'm not worried about being the victim of something."
Both support Second Amendment rights and Simons favors arming teachers.
"I don't think that every teacher should be allowed to have guns, but I definitely do believe that that could be an option," Simons said.
But Hill believes it could put teachers at risk.
"I think if someone goes in a school and they see a person with a gun, they don't know if they're good or bad, they don't know if they've killed others, they're gonna shoot," Hill said.
No one imagined the mass murder of a school shooting could happen there now, with precautions and training. Columbine may be one of the safest schools in America.
CBS News also talked to a teacher who was here 20 years ago. He yelled at students to evacuate and they gave him blank stares. Now in this district, students are trained not just how to respond, but to respond instantly.